1. Field of the Invention
This invention relates to a process for preparing an electrical connecting member to be used in connecting electrically electrical circuit parts to each other, particularly to a process for preparing an electrical connecting member which connects electrically electrical parts to each other with a plurality of electroconductive members held by an electrical insulating material.
2. Related Background Art
In the prior art, as the method for connecting electrically electrical circuit parts to each other, there have been known the wire bonding method, TAB (Tape Automated Bonding) method, etc. However, in these methods, there have been involved the drawbacks that they cannot correspond to the increase of connecting points between the both electrical circuit parts, and are also high in cost. In order to solve such drawbacks, there has been disclosed in the art to connect electrical circuit parts to each other by use of an electrical connecting member having a constitution equipped with a plurality of electroconductive members insulated from each other in an insulating holding member in, for example, Japanese Laid-Open Patent Application Nos. 63-222437, 63-224235, etc.
FIG. 18 illustrates schematically electrical connection between the electrical circuit parts by use of such electrical connecting member, and in the Figure, 31 shows the electrical connecting member, and 32, 33 show the electrical circuit parts to be connected. The electrical connecting member 31 has a plurality of electrical members 34 comprising a metal or an alloy equipped in the holding member 35 comprising an electrically insulating material under the state where the electroconductive members 34 are electrically insulated from each other, and has one end 38 of the electroconductive member 34 exposed on one side of the electrical circuit part 32, and the other end 39 of the electroconductive member exposed on the other side of the electrical circuit part 33 (FIG. 18A). The connecting portion 36 of one electrical circuit part 32 and one end 38 exposed of the electroconductive member 34 are bonded together by alloy formation, while the connecting portion 37 of the other electrical circuit part 33 and the other end 39 exposed of the electroconductive member 34 by alloy formation, thereby connecting electrically electrical circuit parts 32 and 33 to each other (FIG. 18B).
In such electrical connecting member, there are the advantages as shown below.
(1) By making the size of the electroconductive member finer, the connecting portion of the electrical circuit part can be made compact, and also for such reason, the connecting points can be increased. Hence, higher density connection mutually between electrical circuit parts can be done. PA1 (2) Even for electrical circuit parts with different thicknesses, the height of the electrical circuit parts can be made always constant by varying the thickness of the electrical connecting member, whereby multi-layer connection can be done easily to enable high density practical mounting. PA1 (3) By making the protruded height of the electroconductive member to be connected with the connecting portion of the electrical circuit part higher, even one with the connecting portion of the electrical circuit part sunk from the surface can be stably connected, whereby it is possible to connect electrical circuit parts easily with complicated shapes to each other. PA1 (4) Since the heat generated from the electrical circuit parts is radiated through the holding member of the electrical connecting member, the electrical characteristic change is extremely small. Also, because of excellent characteristic of radiation of heat, the influence of thermal fatigue is small and reliability is also high. At this time, the holding member may be also constituted of an insulating material, and its effect becomes still greater if insulating ceramics with high thermal conductivity is used as the holding member. PA1 (5) Since the connecting length in connection mutually between the electrical circuit parts is short, impedance can be reduced and speed-up of electrical parts can be effected. By mixing of a metal, etc. with low dielectric constant into the holding member, the parasitic capacity can be made smaller, and further impedance can be also reduced.
As the process for preparing such electroconductive member as described above for effecting electrical multi-point connection mutually between electrical circuit parts, there is one proposed in U.S. Pat. No. 4,926,549. In the following, this preparation process is described briefly by referring to FIG. 19 showing schematically its steps.
First, a base member 51 comprising a metal sheet such as copper foil, etc. is prepared (FIG. 19A), and on the base member 51 is coated a photosensitive polyimide resin 52 by a spin coater, which is then pre-baked at a temperature around 100.degree. C. (FIG. 19B). The film thickness of the polyimide resin 52 coated is made thicker than the desired film thickness of the holding member in the electrical connecting member prepared in view of the reduction by curing shrinkage. Through a photomask forming a predetermined pattern (not shown), light is irradiated (exposed) on the photosensitive resin 52, followed by developing. At the irradiated portion, the photosensitive resin 52 remains, while at the unirradiated portion, the polyimide resin 52 is removed by the development, whereby a plurality of holes 53 are formed (FIG. 19C). After curing the polyimide resin 52 by elevating the temperature to 200.degree. to 400.degree. C., the base member 51 is dipped in an etchant for the metal to effect etching, thereby forming concavities 54 communicated to the holes 53 on the base member 51 (FIG. 19D). Next, by application of gold plating with the base member 51 as the common electrode, gold 55 is filled into the holes 53, the concavities 54, and gold plating is continued until bumps are formed (FIG. 19E). Finally, the base member 51 is removed by etching to prepare an electrical connecting member 31 (FIG. 19F).
In the electrical connecting member 31 thus prepared, the gold 55 constitutes the electroconductive member 34, and the polyimide resin 52 constitutes the holding member 35. The dimensions of the respective portions in the electrical connecting member 31 are made about 10 .mu.m for the thickness of the polyimide resin 52 (holding member 35), about 20 .mu.m for the diameter of the hole 53 (electroconductive member 34), about 40 .mu.m for pitch, and about several .mu.m for protruded amount at both front and back of the electroconductive member 34.
However, in the preparation process of the prior art proposed in U.S. Pat. No. 4,926,549 as described above, the tasks to be solved as described below remain and there was left room to be further improved.
Since a polyimide resin which becomes the holding member is coated by a spin coater on a base member, particularly when a holding member with a thick film is formed, it is difficult to set the thickness of the holding member with good precision, above all to form a holding member with a uniform film thickness. Also, a polyimide resin is shrinked during thermosetting and has a different coefficient of linear expansion from the base member and therefore has residual stress, whereby the film thickness of the holding member and the pattern dimensions of the hole to be formed can be controlled with difficulty.
Photosensitive polyimide resins include PL-1100, 1200 (Hitachi Chemical Co.,Ltd.), Photoneece (TORAY INDUSTRIES,INC.), Bimide (CIBA GEIGY Ltd.), Selectilax (E. Mark), Lithocoat (Ube Industries, Ltd.), etc., but as compared with the kinds of already imidated commercially available polyimide resins, their number is still very small and range of choice is small. Also, their price is very high.
On the other hand, the film thickness of the photosensitive polyimide resin 52 provided on the metal sheet 51 is large, and yet when a small hole with a small diameter is attempted to be formed as the hole 53, according to the photolithographic technique, a hole with an aspect ratio of 1 or more can be formed with difficulty, therefore not only formation of the thru-hole 53 with good precision is difficult, but also variance occurs in shape of the concavities 54 to be formed by etching on the metal sheet 51, whereby there are involved such tasks that the bump shape cannot be constant, that the stability of the electrical characteristics is low and that the yield is also lowered.
Further, the bump formed on the opposite surface of the photosensitive polyimide resin to the metal sheet is formed by continuing growth of gold plating onto the polyimide resin through the hole perforated through the polyimide resin. However, when the bump height protruded is attempted to be made higher, there is a fear that expanding may occur not only in the height direction but also in the lateral direction, whereby short circuit with adjoining bump may occur. If the interval between the bumps is broadened in order to avoid short circuit, no connection of high density electrical circuit parts (e.g. semiconductor IC chips) can be done. Shortly speaking, if the growth of the protruded bump in the lateral direction is not restricted, it is very difficult to make the bump height higher.
On the other hand, the metal sheet is etched by wet etching to form a concavity, and this concavity becomes shaped in a bump protruded on the metal sheet side. However, since wet etching is isotropic etching, in aiming to make the bump protruded on the metal sheet side still higher, if the concavity is made deeper, expanding also occurs in the lateral direction (side etching), whereby there is a fear that short circuit may occur with adjoining bump. If the interval between the bumps is broadened, it becomes impossible to connect high density electrical circuit parts (e.g. semiconductor IC chips). Shortly speaking, also in this case, there has been involved the task to be solved that it is difficult to make the bump height sufficiently high and yet maintain high density.